Generator Hz Issues

Re: Generator Hz Issues

islandguy wrote: »

This means that a better wave form will also deliver more energy to the device you are powering? In other words my 2000 watt generator that I figured was good for 1k is actually delivering closer to say, 600 watts of power due to the crappy waveform?

That's an approximation, but yes that's basically what you get.

For your application it sounds like an inverter generator would be ideal. I think I'd get a bigger one than what you think you need and use the "eco throttle" feature on it. It should work fine that way because it doesn't sound like you got big surge loads. It's REALLY hard to find a quality conventional generator in the size you're looking for. There's a pretty good selection of inverter-type generators in the size you need, however, and if you size it to run at 50% load most of the time it'll be pretty efficient.
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Chris

Re: Generator Hz Issues

BB. wrote: »

Here is a quick example of how "harmonics" add to create non-standard output waveforms:

Bill, thanks for that info on harmonics. When I design a new wind turbine generator I have to pay attention to pole and coil layout to prevent problems with the second and third harmonic or it will cause the generator to be a "moaner" at certain speeds, which gets very annoying to listen to. Depending on the natural frequency of the tower, the frequency the tower likes to vibrate at can match the generator and turn the whole tower into a giant speaker. Even some commercially built wind turbine generators (like the Skystream 3.7) have problems with harmonics and they're very noisy. When you rectify to DC downstream it doesn't cause any problems with equipment. But the constant vibration in the stator will tend to shake stuff loose on the turbine with time.

Harmonics in a generator winding are unavoidable. But careful design can prevent them from being a problem at the design speed and operating frequency of the generator. Whoever designed the little generator in question in this thread didn't pay attention to the details.

Edit:
Darn it, sometimes I forget to say what I was going to say

Since this little generator is being used primarily for battery charging, why not eliminate the battery charger and put a full-wave bridge on it? Turn it into a DC unit. Then you don't have to run it at rated speed - adjust the throttle to a suitable speed for how many amps you want from it. And if the rectified voltage is too high, use a 150V solar MPPT controller on it. You wouldn't need an expensive controller like a Classic because it will be operating at constant speed, so a regular controller will have no problems "tracking" it. Might have to add a capacitor to the DC output of the rectifier to "smooth" it a bit to prevent damage to the input caps in the controller, being the generator is single phase.
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Chris

Re: Generator Hz Issues

Those little square full-wave bridges that Radio Shack has are fine for 12 or 24V systems, straight up. They are rated at 25 amps and 50V and I've used them on wind turbines before. They are not really 25 amp - more like 15 amps - although they will handle 25 amps for a little bit. You can just parallel two or three of them - hooking the generator's hot to one AC terminal, the neutral to the other, and jumping from one to the other with the hot an neutral to parallel them. You get DC+ and DC- out of the other two terminals, and you parallel those on the DC side. A good heat sink is required for them - a 1/4" aluminum plate works, with a small fan blowing on it.

If you want higher quality bridges you can go to a local motor rewind shop and they can probably get you some 600V single-phase full-wave bridges rated for higher amps, with a heat sink with fins on it. But the Radio Shack ones are only about $5 bucks each, so you're not out a lot of money to try it and if you burn one up, but it seems to work fine, then buy some higher quality bridges.

The bridge is rated on the DC side. So, for instance, if you have a 12V system and want to charge at 1,000 watts, that's about 80 amps @ 12V nominal so you'd have to parallel about 6 of those Radio Shack ones.

I would put a volt meter on the AC output of your generator and idle it down to see how low you can make the voltage go on it, and still have the exciter and AVR maintain output. The rectified DC voltage will be about 1.4x what AC is. So, for instance, if you get it down to 80 volts AC RMS, the peak voltage is really about 110V and that's what you'll get from the rectifier. Hooking it to a battery will "clamp" the voltage to battery voltage. So depending on how aggressive your AVR is will determine whether or not you'll have to use a MPPT controller on it. The MPPT controller would be recommended anyway because it will handle all the charge stages (bulk/absorb/float) and just unload the generator if the power isn't needed. But to try it you'd only need one bridge and a car battery or something to see how the generator reacts to running at slower speed and voltage.
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Chris

Re: Generator Hz Issues

Cariboocoot wrote: »

Funny, I thought he said the gen was putting out 109 Volts RMS. Connecting that to a bridge rectifier rated for 50 Volts doesn't sound like a good idea to me. Oh, you're going to idle down the engine to reduce the Voltage first? Make sure that works before you connect the rectifier.

That 50V is the reverse rating where the diodes won't "leak". I've used those 50V bridges at 140 volts before on a Classic with a wind turbine and they work fine. But you don't want to hook them directly to a 48V battery bank.

Basically, what will happen when you rectify the output of a generator connected to a battery is that the voltage will drop ("clamp") to battery voltage. The AVR will attempt to maintain voltage but it won't be able to under the load and the exciter hopefully will stay working to maintain decent flux in the rotor winding. Different generators react differently to that, but most of them will go down to 80V unloaded before the AVR and exciter stops working. At that voltage, once you "clamp" it by hooking the rectifier to a battery the flux will not change in the exciter winding, but the voltage will drop in the main winding under the load of the battery. To get more watts from it the voltage will have to be allowed to run closer to normal (MPPT controller).

All you can do it try it - the deciding factor is whether or not the AVR and exciter will maintain output so you can idle the engine down using it as a DC unit.

Edit:
The other thing a rectifier does is cause the output of the generator to become more of a square wave - which is actually good for battery charging. The AC sine wave will have a flat top to it when connected to a rectifier that's in turn hooked to a battery due to the "clamp" from the internal resistance of the battery. The diodes in the rectifier will only conduct when the sine wave voltage is above battery voltage. So you get a long "pulse" with a flat top on the voltage (sine wave). The harmonics issues in the generator winding then become a non-issue.
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Chris

Re: Generator Hz Issues

Cariboocoot wrote: »

The rectifier will be subjected to the Voltage peak from the generator, not the RMS. Something to keep in mind when making the adjustment. And fuses will not save you from applying too much Voltage to a device.

That is correct. Those 50V bridges aren't suitable for a 48V system, direct hooked. But on a MPPT controller they will be fine because most MPPT controllers I've seen won't backfeed their source. I was running a bank of 9 of those Radio Shack bridges (three on each phase) on a turbine here with a Classic running at 140VDC for almost two years without any issues.
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Chris

Re: Generator Hz Issues

Cariboocoot wrote: »

Output to an MPPT charge controller on the other hand ... they are designed to be power by PV's which produce very pure DC. There may be a problem with feeding one the somewhat unclean DC from rectified AC.

A Classic will take it with no problem. I've run both single and two phase wind turbines on a Classic and it don't even affect it. Not sure how tough the Outback is.

The limiting factor from the generator when being rectified to DC is the winding size in the head and how many parallel windings they got in it. If it's a 2,000 watt generator at 120V (theoretical) the windings are probably good for about 15 amps continuous. So at 14.5 volts the generator would only output about 200 watts. That's why you need the MPPT controller to get the voltage up to closer to normal.

All a battery charger has in it is a transformer to step high voltage AC down to low voltage AC and then it feeds it thru a couple button diodes. And there's a control board to control charging voltage. That's it - it's pretty simple. The control board is what goes ka-blooie (and sometimes a diode) in a battery charger. The transformer rarely fails in them, although the transformer is a big source of heat.

All you're doing by rectifying the output of the generator directly is eliminating the transformer step and replacing that transformer, and the control board, with a MPPT controller. Rectifying high voltage AC to DC is way more efficient than rectifying low voltage AC to DC the way a battery charger does it. And that's because of the ~1.4V forward voltage drop in your diodes and how much power they dissipate as a result of the product of voltage and amperage (watts).

On the generator side it don't matter if it's permanent magnet type or wound field type. Diodes don't care where the AC comes from or what's producing it, or how "dirty" it is. The peak voltage could be up ~170 VDC (assuming the gen head can actually produce 125VAC). That won't hurt a Classic. I know because when I've had clippers evaporate in high wind I've fed well over 200 into a Classic 150 with wind turbines and it never hurt it. Again, not sure how tough a Outback is in that respect.

BUT - I'm sure the AVR on that generator has an adjustment screw (most of 'em do) and you can usually turn 'em down as low as 100-105VAC @ 60Hz. And most gen heads with an exciter winding and AVR will produce voltage with the engine idling at 2,200-2,500 rpm with no problem. Brushless heads with a PM exciter won't produce any voltage at engine idle speed, however. But I don't think that's a brushless generator head on that little genset.

So, basically, if I had that generator and it's primary duty was battery charging (either thru an inverter or a plug-in battery charger), and I wasn't happy with its output because it's a cheap unit - that's what I'd do with it. I'd have it all set up and working inside of an hour and a half with a Classic 150 to try it. I even found a couple of those Radio Shack bridges in a drawer in the shop - never know what you might need one for :cool:

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The totally best option is to still buy a nice little inverter gen because this sounds like an ideal application for one. But if you don't want to throw that cheap generator out and still might want to find a use for it, with a few modifications and fiddling around I'm 95% certain you can turn it into an off-grid DC genset.
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Chris